Cascade cryogenic thermoelectric cooler for cryogenic and room temperature applications

ABSTRACT

A cascade thermoelectric cooler designed to cool to cryogenic temperatures of 30 to 120 K. integrates high performance\high-ZT Bi x Sb 2−x Te 3  and Bi 2 Te 3−x Se 3  based super-lattice-structure thin-film thermoelectric devices with a bulk-material based thermoelectric cooler including plural cascaded cold stages with each successive cascaded cold stage able to cool to a progressively lower temperature. Each cold stage in the bulk-material thermoelectric cooler includes a heat source plate, a heat sink plate, a p-type thermoelectric, and a n-type thermoelectric. Moreover, the thin-film thermoelectric cooler can have multiple stages in which each stage contains a heat source plate, a heat sink plate, a p-type super-latticed thermoelectric element, and a n type super-latticed thermoelectric element. By bonding an output heat source plate on the thin-film thermoelectric cooler to an input heat sink plate on the bulk-material thermoelectric cooler, the integration of the thin-film thermoelectric with the bulk-material-based thermoelectric yields a cascade thermoelectric cooler wherein the bulk-material-based thermoelectric cooler cools to 160 K. and the thin-film thermoelectric device cools to cryogenic temperatures between 70 and 120 K. Another level of thin-film super-lattice integration can achieve temperatures of 30 K. Alternatively, the integration of a high ZT thin-film superlattice thermoelectric cooler on a multi-staged bulk-material-based thermoelectric cooler can produce a higher performance non-cryogenic cooler.

CROSS REFERENCE TO RELATED DOCUMENTS

[0001] This application claims benefit of priority to U.S. ProvisionalApplication No. 60/190,924 filed in the United States Patent andTrademark Office on Mar. 21, 2000, the entire disclosure of which isincorporated herein by reference. This application is related to U.S.Ser. No. 09/381,963,“Thin Film Thermoelectric Device and FabricationMethod of Same”, filed Sep. 30, 1999 and recently allowed, the entiredisclosure of which is incorporated herein by reference. Thisapplication is also related to U.S. Provisional Application No.60/253,743, “Spontaneous Emission Enhanced Heat Transport Method andStructures for Cooling, Sensing, and Power Generation”, filed Nov. 29,2000, the entire disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a cascade cryogenic thermoelectriccooler and a method of manufacturing the same. The cascade cryogenicthermoelectric cooler integrates high coefficient of performancethin-film super-lattice devices with cascaded bulk-material-basedthermoelectric devices to enable cooling to cryogenic temperatures suchas 30-120 K.

[0004] 2. Discussion of the Background

[0005] Solid-state thermoelectric cooling to cryogenic temperaturesbetween 70 and 120 K. will improve the performance of electronics andsensors such as for example RF receiver front-ends, infrared (IR)imagers, ultra-sensitive magnetic signature sensors, and superconductingelectronics based on high-_(c) (100 to 120 K.) superconductingmaterials.

[0006] Today, bulk thermoelectric materials based on p-Bi_(x)Sb_(2−x)Te₃and n-Bi₂Te_(3−x)Se_(x) do not have a sufficient figure-of-merit (ZT) ora coefficient of performance (COP) to achieve cryogenic temperatures.For example, a commercial 6-stage Melcor thermoelectric cooler (Melcor,Trenton, N.J.) with a COP of about 0.028 can only approach a cold-sidetemperature of about 167 K. for a hot-side temperature of 300 K.Similarly a 6-stage Marlow thermoelectric cooler (Marlow Industries,Dallas, Tex.) can approach a temperature of about 165 K. with a COP of0.026.

[0007] The principle reason that thermoelectric devices with a hot-sideof 300 K. based on bulk p-Bi_(2−x)Sb_(x)Te₃ and bulk n-Bi₂Te_(3−y)Se_(y)can not approach cryogenic temperatures is that the ZT values of bulkmaterials drop as the temperature lowers. The figure of merit drops atlower temperatures because of a higher thermal conductivity as well as alower Seebeck coefficient.

[0008] One bulk-material which does not have low ZT values at lowertemperature is BiSb A BiSb device could be stacked on top of a coolermade from bulk p-Bi_(2−x)Sb_(x)Te₃ and bulk n-Bi₂Te_(3−y)Se_(y).However, for BiSb to offers a reasonable ZT, in order to achievecryogenic temperatures, a magnetic field must also be used; this is notpractical in most applications. Furthermore, both n- and p-typeconducting BiSb materials are not achievable.

[0009] In essence, there are no set of known bulk thermoelectricmaterials (certainly not devices) that have sufficient ZT (and COP indevices) between 85 and 300 K. to achieve cryogenic refrigeration.

[0010] In contrast to bulk materials, the thermal conductivity ofsuperlattice structures decreases at lower temperatures. A variety ofprocesses in superlattice structures such as for example mini-bandconduction, lack of alloy scattering, and interface-carrier-scatteringapparently better preserve reasonable Seebeck coefficients at lowertemperatures. Thus, superlattice materials are expected to have at lowertemperatures higher ZT values than bulk-materials, and devices made fromsuperlattice materials are expected to have higher COP. Despite thehigher ZT of superlattice thin-film materials, thin film cryogenicthermoelectric coolers are not available. Integration of a large numberof superlattice thin-film device stages necessary to achieve thetemperature difference between room and cryogenic temperatures presentscomplications which are beyond the maturity of superlatticethermoelectric devices, presently limited by thermal mismatch andtemperature gradient issues and also practically limited by the highcost of thin-film superlattice materials.

[0011] Thus, an all-thermoelectric cryogenic cooler, implying theadvantages of solid-state reliability and without additionalmechanical/or other forms of cooling, is not available.

SUMMARY OF THE INVENTION

[0012] Accordingly, one object of the present invention is to provide acascade cryogenic thermoelectric cooler integrating a bulk-materialbased thermoelectric cooler with a super-latticed thermoelectric cooler.The bulk-material based thermoelectric cooler is configured with acascade of multiple stages with each stage configured to cool toprogressively lower temperatures, and the super-latticed thermoelectriccooler is interfaced to the bulk material device thermoelectric cooler.

[0013] Another object of the present invention is to provide a cascadecryogenic thermoelectric cooler which can approach a cold sidetemperature of 85 K.

[0014] Still another object of the present invention is to interface asuper-lattice thin film thermoelectric cooler with a bulk-material-basedthermoelectric cooler such that the bulk-material-based thermoelectriccooler reduces the hot-side temperature of the super-lattice thin filmthermoelectric cooler to significantly below 300 K., for example between170-200 K., wherein super-lattice materials relying on the thermalconductivity reduction due to phonon scattering at the super-latticeinterfaces will be more efficient.

[0015] A further object of the present invention is to reduce thethermal mismatch and temperature gradients imposed on a cascade ofsuper-lattice thin-film coolers.

[0016] Another object of the present invention is to provide athermoelectric cooler wherein the potentially expensive super-latticetechnology is utilized only for achieving cryogenic or near-cryogenictemperatures and thus provides a cost-effective cryogenic cooler.

[0017] Still another object of the present invention is to provide anintegrated thermoelectric cooler in which high performance/high ZTsuperlattice structure thin-film thermoelectric devices could be used tomore efficiently cool than a thermoelectric cooler using onlybulk-materials.

[0018] These and other objects are achieved according to the presentinvention by providing a novel cascade thermoelectric cooler designed tocool to cryogenic temperatures of 30 to 120 K. The cascadethermoelectric cooler integrates high performance\high-ZTBi_(x)Sb_(2−x)Te₃ and Bi₂Te_(3−x)Se_(x)-based super-lattice-structurethin-film thermoelectric devices with a bulk-material basedthermoelectric cooler including plural cascaded cold stages with eachsuccessive cascaded cold stage able to cool to a progressively lowertemperature. Each cold stage in the bulk-material thermoelectric coolerincludes a heat source plate, a heat sink plate, p-type thermoelectricelements, and n-type thermoelectric elements. Moreover, the thin filmthermoelectric cooler can have multiple stages which each stage containsa heat source plate, a heat sink plate, p-type super-latticedthermoelectric elements, and n type super-latticed thermoelectricelements. By attaching an output heat source plate on the thin-filmthermoelectric cooler to an input heat sink plate on the bulk-materialthermoelectric cooler, the integration of the thin film thermoelectricwith the bulk-material-based thermoelectric yields a cascadethermoelectric cooler wherein the bulk-material-based thermoelectriccooler cools to 170-200 K. and the thin-film thermoelectric device coolsto cryogenic temperatures between 70 and 120 K. Another level ofthin-film super-lattice integration can achieve temperatures near 30 K.

[0019] According to one aspect of the present invention, the cascadethermoelectric cooler is utilized to cool superconducting coils inelectric motors or generators. The cascade cooler is either integrateddirectly in contact with the superconducting coils or mounted to asub-77 K. transfer coupling in thermal contact with the superconductingcoils. The cascade cooler either cools through multiple stages from nearroom temperature to cryogenic temperatures or cools from liquid nitrogentemperatures (i.e. 77 K.) to cryogenic temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] A more complete appreciation of the invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

[0021]FIG. 1 is schematic of a thin-film thermoelectric cooler usingsuperlattice films cascaded on to a conventional bulk thermoelectriccooler;

[0022]FIG. 2 is schematic of a one-stage thin-film cooler usingsuperlattice thin-film materials;

[0023]FIG. 3 is a schematic of a two-stage thin-film cooler usingsuperlattice thin-film materials;

[0024]FIG. 4 is a schematic of an electric apparatus utilizingsuperconducting coils being cooled by a liquid helium refrigerator;

[0025]FIG. 5 is a schematic of an electric apparatus, according to thepresent invention, utilizing superconducting coils being cooled by aliquid-nitrogen assisted thermoelectric cascade cooler;

[0026]FIG. 6 is a schematic of an electric apparatus, according to thepresent invention, utilizing superconducting coils being cooled by athermoelectric cascade cooler coupled between room temperature andcryogenic temperatures of the superconducting coils;

[0027]FIG. 7 is a schematic of an electric apparatus, according to thepresent invention, utilizing superconducting coils being cooled by anintegral thermoelectric cascade cooler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Referring now to the drawings, wherein like reference numeralsdesignate identical or corresponding parts throughout the several views,and more particularly to FIG. 1 thereof, there is shown a schematic of athin-film thermoelectric cooler using superlattice films cascaded onto aconventional bulk thermoelectric cooler. Thus, the cooler according tothe present invention integrates a super-lattice-structure thin filmthermoelectric device with a conventional multi-stagedbulk-material-based thermoelectric cooler having a progressively highernumber bulk-material-based thermoelectric devices.

[0029] While the present invention is not limited to any particulardesign of the bulk or thin-film thermoelectric coolers, either in thetype of materials used or specific device design. FIG. 1 illustrates thebasic structure of the cascade cooler which has a multi-stagedbulk-material based thermoelectric cooler 1 onto which a thin-filmthermoelectric cooler 2 is interfaced. FIG. 1 shows that each stage 3,4, 5, 6, 7, 8 of the bulk-material-based thermoelectric cooler includesa heat source plate 9, a heat sink plate 10 operating at an elevatedtemperature with respect to the heat sink plate, at least one pair of an-type bulk-material thermoelectric element 11 and a p-typebulk-material thermoelectric element 12 connected electrically in seriesand disposed between the heat source and sink plates with each pair ofbulk-material thermoelectric elements electrically connected in series.Each stage from ambient temperature results in a progressively lowertemperature until near cryogenic temperatures are reached.

[0030] Thus in one preferred embodiment, the cascade thermoelectriccooler of the present invention utilizes a bulk-material-basedthermoelectric cooler having multiple cold stages with each hotter stagehaving a progressively higher number of bulk-material-basedthermoelectric devices to compensate for internal heat generation withinthe individual thermoelectric devices.

[0031] In another preferred embodiment, the cascade thermoelectriccooler of the present invention utilizes in each cold stage of thebulk-material-based thermoelectric cooler p-type Bi_(x)Sb_(2−x)Te₃, andn-type Bi₂Te_(3−x)Se₃ thermoelectric elements. Other bulk materials suchas CdBi₄Te₆ could be used for the low temperature stages within thebulk-material-based cooler.

[0032] In a still another preferred embodiment, the cascadethermoelectric cooler of the present invention integrates ahigh-performance/high-ZT Bi₂Te₃-based superlattice structured thin-filmthermoelectric cooler onto a conventional bulk-material-basedthermoelectric cooler. This integration provides high ZT materials inthe cold stages of the cascade thermoelectric cooler. ZT values forsuperlattice materials at low temperatures (200 K. to 77 K.) are likelyto be between 1-2. In contrast, bulk materials have ZT in the range of0.5 or less. Consequently, superlattice material devices are expected tooffer a significantly larger COP compared to current state-of-the-artbulk material devices.

[0033]FIG. 2 shows a schematic of a single-stage thin-film cooler usingsuperlattice thin-film materials which is described in pending U.S.patent application Ser. No. 09/147,199, herein incorporated byreference. Such high-performance/high-ZT Bi₂Te₃-based superlatticestructured thin-film thermoelectric materials are discussed elsewhere inR. Venkatasubramanian et al., Appl. Phys. Lett., 75, 1104 (1999), R.Venkatasubramanian, Phys. Rev. B, 61, 3091 (2000), and R.Venkatasubramanian and T. S. Colpitts, in Thermoelectric Materials—NewDirections and Approaches, Ed. by T. M. Tritt et. al, MRS SymposiaProceedings No. 478, (MRS, Pittsburgh, 1997), p.73. The contents ofthese references are incorporated herein by reference. FIG. 2 shows thatthe single-stage thin-film thermoelectric device includes a heat sourceplate 13, a heat sink plate 14 operating at an elevated temperature withrespect to the heat sink plate, pairs of n-type super-latticethermoelectric elements 15 and p-type super-lattice thermoelectricelements 16 connected electrically in series and disposed between theheat source and sink plates. As shown in FIG. 2, electrodes 17 areprovided to the thin film thermoelectric device to supply electricalcurrent for cooling.

[0034] In another preferred embodiment, an electrically insulating film18 is provided between the thermoelectric elements 15, 16 and the heatplates 13, 14, if the heat plates 13, 14 are electrically conducting.

[0035] In a preferred embodiment, the superlattice structured thin-filmthermoelectric coolers utilize p-type super-latticed thermoelectricelements with a structure having alternate layers of Bi₂Te₃ and Sb₂Te₃.Other superlattice materials could be used such as alloys ofBi_(x)Sb²⁻Te₃ and Bi_(y)Sb_(3−y)Te₃, p-type CdSb/ZnSb, and p-typeSi_(x)Ge_(1−x)/Si_(y)Ge_(1−y).

[0036] In one preferred embodiment, the superlattice structuredthin-film thermoelectric coolers utilize n-type super-latticedthermoelectric elements with a structure having alternate layers ofBi₂Te_(3−x)Se_(x) and Bi_(y)Te_(3−y)Te_(y). Other superlattice materialscould be used, such as for example n-type Si_(x)Ge_(1−x)/Si_(y)Ge_(1−y).

[0037] In another preferred embodiment, the cascade thermoelectriccooler of the present invention, as shown in FIG. 3, includes amulti-stage thin-film thermoelectric cooler integrated onto a cascadedmulti-stage bulk-material-based thermoelectric cooler for achievinglower cryogenic temperatures (30 K.-70 K.), for achieving higher COP, orfor reducing the temperature gradient in each stage of the thin-filmcooler. Specifically, FIG. 3 shows a two-stage thin-film cooler usingsuperlattice thin-film materials in which a first stage 19 contains 2pairs of superlattice thin-film thermoelectric devices and a secondstage 20 contains 4 pairs of superlattice thin-film thermoelectricdevices. Other embodiments of such a device can involve variations insuperlattice spacing, bandgaps, or superlattice components, as is knownby those skilled in the art of superlattice engineering, to optimize theperformance at various temperature regimes.

[0038] More specifically, the thin-film thermoelectric cooler can befabricated from p-type Bi₂Te₃/Sb₂Te₃ and n-typeBi₂Te_(3−x)Se_(x)/B₂Te_(3−y)Se_(y) superlattice system using MOCVDtechnologies to form alternating layers of Bi₂Te₃ and Sb₂Te₃ orBi₂Te_(3−x)Se_(x) and Bi₂Te_(3−y)Se_(y) at superlattice periods rangingfrom 40 to 70 Å. The integration of electrical and thermalinterconnections of p and n type thin-film super-lattice materials intoa thin-film thermoelectric cooler is shown in FIG. 2 and described inthe aforementioned co-pending U.S. patent application Ser. No.09/147,199.

[0039] Integration of the thin-film thermoelectric cooler to the cascadeof bulk-material-based thermoelectric elements can be accomplished, forexample, by forming the superlattice structure on Si, polycrystallinediamond, SiC, BeO, or other high thermal conductivity substrates andthen bonding the substrate with the formed superlattice structure to abulk-material-based thermoelectnc cooler using bonding techniques suchas for example discussed in R. Venkatasubramanian et al., Appl. Phys.Lett. 60, 886(1992), herein incorporated by reference, or Qin-Yin Tonget al. Adv. Mat. 17, 1409(1999), herein incorporated by reference.

[0040] Accordingly, the cascaded thermoelectric cooler of the presentinvention offers a number of advantages which can not be realized bythermoelectric coolers composed of only thin-film superlatticethermoelectric devices or bulk-material-based thermoelectric devices.

[0041] First, as noted above, a 6-stage Melcor bulk-material-basedthermoelectric cooler with a hot side temperature of 300 K. can notachieve a cold-side temperature of less than 160 K. Whereas the cascadecooler of the present invention can approach cold-side cryogenictemperatures as low as 30-120 K., for a hot-side temperatures near roomtemperature (i.e., near 300 K.).

[0042] Second, a single-stage superlattice thin-film thermoelectriccooler, even with a ZT of 2.65 in the temperature range of 300 K. andbelow, can not achieve a cold-side temperature less than 160 K. with ahot-side temperature of 300 K. Thus, cryo-cooling (i.e., 30 K. to 120K.) is not possible with a single stage thermoelectric device. However,in the present invention a single-stage superlattice thin-film coolerintegrated onto a commercially available cascaded 6-stage bulk coolercan achieve cryogenic temperatures.

[0043] Third, the cascade thermoelectric cooler of the present inventionavoids a multi-staged (three or four) superlattice thin-film cooler forachieving cryogenic temperatures. Thus, the cascade thermoelectriccooler of the present invention limits use of the potentially expensivesuperlattice technology to the critical “cryogenic” or near cryogenicstages.

[0044] Fourth, utilization of thin-film superlattice materials in thecryogenic or near cryogenic stages of the cascade cooler of the presentinvention will likely to be more appropriate than relying on thin-filmsuperlattice materials in non-cryogenic stages. The thermal conductivityreduction from phonon scattering at superlattice interfaces [R.Venkatasubramanian and T. S. Colpitts, in Thermoelectric Materials—NewDirections and Approaches, Ed. by T. M. Tritt et. al, MRS SymposiaProceedings No. 478, (MRS, Pittsburgh, 1997), p.73] is apparently moreeffective at temperatures less than 300 K. than above 300 K. [S. M. Lee,D. G. Cahill, and R. Venkatasubramanian, Appl. Phys. Lett., 70, 2957(1997)].

[0045] Fifth, integration of a single-stage superlattice thin-filmcooler onto a cascade of bulk-material-devices, as described above, mayreduce the demands placed on thin-film superlattice thermoelectrictechnology in the area of thermal mismatch and temperature gradientissues. This reduced demand can be inferred from the fact that the bulkcooler maintains a significant portion of the total temperaturedifferential between the cryogenic side and the hot side.

[0046] Sixth, it is anticipated that a high ZT thin-film cooler (singleor multiple stages) integrated onto a bulk cooler can offer a higher COPas compared to a bulk cooler in cooling systems intended fornon-cryogenic temperatures (such as 200 to 250 K.). Thus, the inventiondescribed, while likely to be most useful for achieving an all-solidstate thermoelectric cryogenic cooling, is also useful for non-cryogeniccooling applications.

[0047] One application for the cascade thermoelectric coolers of thepresent invention is in cooling superconducting coils found in electricmotors and generators. Large power industrial motors and generators canbenefit significantly from the use of superconducting coils. Theapplication of superconducting coils to industrial motors and generatorswould reduce substantially the rotor ohmic losses (i.e., I²R). Thereduction in I²R loss will more than compensate for the extra powerrequired for refrigeration of the superconducting coils. While somehigh-temperature superconductors are superconducting at liquid nitrogentemperatures of 77 K., in the presence of magnetic fields (such as inelectric motors or generators), the current carrying ability of theseliquid nitrogen superconductors is deteriorated. Thus, more expensiveliquid helium (at temperatures of 4.2K.) is utilized to keep thesuperconducting coils at 30 to 50 K., where in the presence of amagnetic field the current carrying capability is not deteriorated.

[0048]FIG. 4 is a schematic of an electric apparatus 40 (e.g. anelectric motor or generator) containing superconducting coils 42. Adrive shaft 44 of the apparatus is mounted inside an internal jacket 46a contained in a frame 46 b. The internal jacket is typically a vacuumjacket to minimize heat conduction from the outside environment to thesuperconducting coils. The internal jacket can include adsorbenttrapping material to capture moisture from the vacuum of the internaljacket. The superconducting coils 42 are supported by a coil supportstructure 48. For motor applications, an exciter 50 induces current inthe superconducting coils 42 to drive the motor 40. A liquid heliumrefrigeration system 52 provides liquid helium to cool the coils 42 viaa cryogenic transfer coupling 54. The cryogenic transfer coupling 54 isthermally in contact with the superconducting coils 42.

[0049]FIG. 5 is a schematic of an electric apparatus, according to thepresent invention, utilizing a liquid-nitrogen assisted thermoelectriccascade cooling system 56 to cool the superconducting coils 42. Theliquid-nitrogen assisted thermoelectric cooling system 56 depicted inFIG. 5 is an attractive alternative to expensive liquid helium coolingsystems. The liquid-nitrogen-assisted thermoelectric cooling system 56includes a liquid nitrogen refrigeration system 58, a 77 K. transfercoupling 60, and a cascade thermoelectric module 62 similar to themulti-staged thermoelectric cooler 1 shown in FIG. 1. In thisembodiment, the cascade thermoelectric module 62 utilizes a hot sidemaintained at 77 K. with liquid nitrogen and a cold side at sub-77 K.temperatures. Further, the thermoelectric cooling system 56 includes asub-77 K. transfer coupling 64 in thermal connection with thesuperconducting coils 42. The transfer couplings 60 and 64, according tothe present invention, are thermal link devices similar to thosedisclosed in U.S. Pat. No. 6,164, 077, the entire contents of which areincorporated herein by reference. Alternatively, the transfer couplings60 and 64, according to the present invention, are rotatablegaseous-helium transfer coupling devices similar to those disclosed inU.S. Pat. No. 5,513,498, the entire contents of which are incorporatedherein by reference. Further, in FIG. 5, a DC current is delivered (inmotor applications) from a brushless exciter 50 which energizes thehigh-temperature superconducting coils and provides power to thethermoelectric cascade module 62 via a power feed 66.

[0050]FIG. 6 depicts another embodiment of the present invention inwhich an all-thermoelectric cooling system 70 is utilized requiring noliquid nitrogen. In the all-thermoelectric cooling system 70, a hot side72 of a thermoelectric cascade cooler 74 is near room temperature (i.e.˜300 K.), and a cold side 76 of the thermoelectric cascade cooler 74 ismaintained at cryogenic temperatures (e.g. 30 K. to 50 K.). In thisembodiment, the cold side 76 is a sub-77 K. transfer coupling to thesuperconducting coils 42. The all-thermoelectric cooling system 70 ismore reliable and easier to implement due to the absence of systemrefrigerants. In FIG. 6, DC power to the cascade cooler 74 comes (inmotor applications) from the exciter 50 or from a separate DC powersupply.

[0051]FIG. 7 depicts still another embodiment of the present invention.In this embodiment, a cascade thermoelectric cooler 80 is integratedonto the superconducting coils 42 of the electric motor. Thus, thecascade thermoelectric cooler 80 provides on-spot cooling to thesuperconducting coils. On spot cooling is expected to be more efficientthan transfer cooling permitting applicability to even smaller powerindustrial motors and generators. In this embodiment, heat is pumpedfrom the cascade thermoelectric cooler 80 to an outside heat-sink 82 viaheat transfer fluids pumped in a circulation system 84. Alternatively,enhanced radiative heat transfer as disclosed in the related applicationU.S. Provisional No. 60/253,743, the entire contents of which have beenincorporated by reference, is utilized to couple heat from the cascadethermoelectric cooler 80 to the outside. Power feeds 86 supply power tothe thermoelectric cooler 80.

[0052] Other configurations of motor or generator design known to thosein the art, such as for example motors utilizing superconducting coilson either rotor or stator coil assemblies, could utilize the cascadethermoelectric coolers of the present invention to reduce ohmic lossesin the coils and produce a more efficient motor or generator. In thosedesigns, the cascade coolers as in the previous embodiments will be inthermal connection with the superconducting coils and will either coolthe coils to cryogenic temperatures with or without liquid nitrogenassisted cooling.

[0053] Numerous modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patents ofthe United States is:
 1. A cascade thermoelectric cooler comprising: abulk-material-based thermoelectric cooler including plural cascaded coldstages with each successive cascaded cold stage configured to cool to aprogressively lower temperature, and a super-latticed thermoelectriccooler interfaced to said bulk-material-based thermoelectric cooler. 2.The cooler of claim 1, wherein the bulk-material-based thermoelectriccooler is configured such that said each cold stage of saidthermoelectric cooler is in thermal contact with a successive adjacentcold stage and said successive cold stages have a progressively highernumber of bulk-material-based thermoelectric elements.
 3. The cooler ofclaim 2, wherein said each successive cascaded cold stage of thebulk-material-based thermoelectric cooler comprises: a heat sourceplate; a heat sink plate operating at an elevated temperature withrespect to the heat source plate; and at least one pair of a n-typebulk-material thermoelectric element and a p-type bulk-materialthermoelectric element connected electrically in series and disposedbetween the heat source and sink plates, each pair of bulk-materialthermoelectric elements electrically connected in series.
 4. The coolerof claim 3, wherein the p-type bulk-material thermoelectric elementcomprises p-Bi_(x)Sb_(2−x)Te₃.
 5. The cooler of claims 3 or 4, whereinthe n-type bulk-material thermoelectric element comprisesn-Bi₂Te_(3−x)Se_(x).
 6. The cooler of claim 1, wherein thesuper-latticed thermoelectric cooler comprises a single staged thin-filmthermoelectric device.
 7. The cooler of claim 6, wherein thesingle-staged thin-film thermoelectric device comprises: a heat sourceplate; a heat sink plate operating at an elevated temperature withrespect to the heat source plate; and at least one pair of a n-typesuper-lattice thermoelectric element and a p-type super-latticethermoelectric element connected electrically in series and disposedbetween the heat source and sink plates.
 8. The cooler of claim 7,wherein the p-type and n-type super-latticed thermoelectric elementscomprise materials with ZT greater than 0.5.
 9. The cooler of claim 8,wherein the materials with ZT greater than 0.5 are selected from a groupof superlattice materials consisting of Bi_(x)Sb_(2−x)Te₃,Bi₂Te_(3−x)Se₃, CdSb, ZnSb, and Si_(x)Ge_(1−x).
 10. The cooler of claim8, wherein the n-type super-latticed thermoelectric element comprises astructure having alternate layers of Bi₂Te_(3−x)Se_(x) and Bi₂Te_(3−y)Sewith superlattice periods preferably in the range of 40 to 80 Angstroms.11. The cooler of claim 8, wherein the p-type super-latticedthermoelectric element comprises a structure having alternate layers ofBi_(x)Sb_(2−x)Te₃ and Bi_(y)Sb_(2−y)Te₃ with superlattice periodspreferably in the range of 40 to 80 Angstroms.
 12. The cooler of claim8, wherein the n-type super-latticed thermoelectric element comprises astructure having alternate layers of Si_(x)Ge_(1−z) and Si_(y)Ge_(1−y).13. The cooler of claim 8, wherein the p-type super-latticedthermoelectric element comprises a structure having alternate layers ofSi_(x)Ge_(1−x) and Si_(y)Ge_(1−y).
 14. The cooler of claim 7, wherein atleast one of the heat sink plate and the heat source plate comprises asubstrate for growth of the super-latticed thermoelectric elements. 15.The cooler according to claims 14, wherein the substrate is selectedfrom a group of materials consisting of Si, SiC, polycrystallinediamond, and BeO.
 16. The cooler according to claim 15, wherein thesubstrate includes an electrically insulating film configured to provideelectrical isolation between the substrate and the at least one pair ofsuper-lattice thermoelectric elements.
 17. The cooler of claim 6,wherein the bulk-material-based thermoelectric cooler is configured tocool to between 170 and 200 K. and the single staged thin-filmthermoelectric device is configured to cool to cryogenic temperaturesbetween 30 and 140 K.
 18. The cooler of claim 6, wherein thebulk-material-based thermoelectric cooler is configured to cool tobetween 200 and 300 K. and the single staged thin-film thermoelectricdevice is configured to cool to non-cryogenic temperatures between 140and 200 K.
 19. The cooler of claim 6, wherein the bulk-material-basedthermoelectric cooler is configured to cool to between 300 and 400 K.and the single staged thin-film thermoelectric device is configured tocool to non-cryogenic temperatures between 200 and 300 K.
 20. The coolerof claim 1, wherein the super-latticed thermoelectric cooler comprises amulti-staged thin-film thermoelectric device.
 21. The cooler of claim20, wherein the multi-staged thin-film thermoelectric device comprises aseries of thin-film stages, each said thin-film stage comprising: a heatsource plate; a heat sink plate operating at an elevated temperaturewith respect to the heat source plate; at least one pair of a n-typesuper-lattice thermoelectric element and a p-type super-latticethermoelectric element connected electrically in series and disposedbetween the heat source and sink plates.
 22. The cooler of claim 21,wherein the p-type and n-type super-latticed thermoelectric elementscomprise materials with ZT greater than 0.5.
 23. The cooler of claim 22,wherein the materials with ZT greater than 0.5 are selected from a groupof superlattice materials consisting of Bi_(x)Sb_(2−x)Te₃, Bi₂Te³⁻Se₃,CdSb, ZnSb, and Si_(x)Ge_(1−x).
 24. The cooler of claim 22, wherein thep-type superlatticed thermoelectric element comprises a structure havingalternate layers of Bi_(x)Sb_(2−x)Te₃ and Bi_(y)Sb_(2−y)Te₃.
 25. Thecooler of claim 22, wherein the n-type super-lattice thermoelectricelement comprises a superlattice structure having alternate layers ofBi₂Te_(3−x)Se_(x) and Bi₂Te_(3−y)Se_(y).
 26. The cooler of claim 22,wherein the n-type super-latticed thermoelectric element comprises astructure having alternate layers of Si_(x)Ge_(1−x) and Si_(y)Ge_(1−y).27. The cooler of claim 22, wherein the p-type super-latticedthermoelectric element comprises a structure having alternate layers ofSi_(x)Ge_(1−x) and Si_(y)Ge_(1−y).
 28. The cascade cryogenicthermoelectric cooler of claim 21, wherein at least one of the heat sinkplate and the heat source plate comprises a substrate for growth of thesuper-latticed thermoelectric elements.
 29. The cooler according toclaims 28, wherein the substrate is selected from a group of materialsconsisting of Si, SiC, polycrystalline diamond, and BeO.
 30. The coolerof claim 29, wherein the substrate includes an electrically insulatingfilm configured to provide electrical isolation between the substrateand the at least one pair of super-lattice thermoelectric elements. 31.The cooler of claim 20, wherein the bulk-material-based thermoelectriccooler is configured to cool to between 170 and 200 K. and themulti-staged thin-film thermoelectric device is configured to cool tocryogenic temperatures between 30 and 70 K.
 32. The cooler of claim 20,wherein the bulk-material-based thermoelectric cooler is configured tocool to between 200 and 300 K. and the multi-staged thin-filmthermoelectric device is configured to cool to non-cryogenictemperatures between 140 and 200 K.
 33. The cooler of claim 20, whereinthe bulk-material-based thermoelectric cooler is configured to cool tobetween 300 and 400 K. and the single staged thin-film thermoelectricdevice is configured to cool to non-cryogenic temperatures between 200and 300 K.
 34. A method of manufacturing a cascade thermoelectriccooler, comprising: attaching a superlattice thermoelectric device to aninput heat source plate and an output heat sink plate; and bonding theoutput heat sink plate on the super-latticed thermoelectric device to abulk-material thermoelectric cooling device having plural cascaded coldstages with each successive cascaded cold stage configured to cool to aprogressively lower temperature.
 35. The method of 34, wherein the stepof attaching comprises the step of: bonding the input heat source plateand the output heat sink plate to the super-latticed thermoelectricdevice.
 36. The method of 34, wherein the step of attaching comprisesthe steps of: fabricating a super-latticed thermoelectric device on asubstrate configured to be the input heat source plate for thesuper-latticed thermoelectric device; and bonding the output heat sinkplate to a side of the super-latticed thermoelectric device oppositesaid output heat sink plate.
 37. The method of 36, wherein the step offabricating a super-latticed thermoelectric device on a substrateserving as the input heat sink plate comprises a step of: fabricatingsaid super-lattice thermoelectric device on a substrate selected from agroup of materials consisting of Si, SiC, polycrystalline diamond, andBeO.
 38. The method of 37, wherein the step of fabricating saidsuper-lattice thermoelectric device on a substrate selected from a groupof materials comprises a step of: providing a thin electricallyinsulating film on said selected substrate.
 39. The method of 34,wherein the step of attaching comprises the steps of: fabricating asuper-latticed thermoelectric device on a substrate configured to be theoutput heat sink plate for the super-latticed thermoelectric device; andbonding the input heat source plate to a side of the super-latticedthermoelectric device opposite said output heat sink plate.
 40. Themethod of 39, wherein the step of fabricating a super-latticedthermoelectric device on a substrate serving as an output heat sinkplate comprises a step of: fabricating said super-lattice thermoelectricdevice on a substrate selected from a group of materials consisting ofSi, SiC, polycrystalline diamond, and BeO.
 41. The method of 40, whereinthe step of fabricating said super-lattice thermoelectric device on asubstrate selected from a group of materials comprises a step of:providing a thin electrically insulating film on said selectedsubstrate.
 42. The method of 34, wherein the step of attaching asuperlattice thermoelectric device to an input heat source plate and anoutput heat sink plate comprises a step of: attaching said input heatsource and output heat sink plates to a super-lattice thermoelectricdevice having at least one pair of a n-type super-lattice thermoelectricelement and a p-type super-lattice thermoelectric element connectedelectrically in series and disposed between the heat source and sinkplates.
 43. The method of 34, wherein the step of attaching asuperlattice thermoelectric device to an input heat source plate and anoutput heat sink plate comprises a step of: attaching said input heatsource and output heat sink plates to a super-lattice thermoelectricdevice having n-type super-latticed thermoelectric elements withalternate layers of Bi₂Te_(3−x)Se_(x) and Bi₂Te_(3−y)Se_(x) and p-typesuper-latticed thermoelectric elements with alternate layers ofBi_(x)Sb_(2−x)Te₃ and Bi_(y)Sb_(2−y)Te₃.
 44. The method of 34, whereinthe step of attaching a superlattice thermoelectric device to an inputheat source plate and an output heat sink plate device comprises a stepof: attaching said input heat source and output heat sink plates to asuper-lattice thermoelectric device having thermoelectric elements witha ZT greater than 0.5.
 45. The method of 34, wherein the step ofattaching a superlattice thermoelectric device to an input heat sourceplate and an output heat sink plate comprises a step of: attaching saidinput heat source and output heat sink plates to a super-latticethermoelectric device having super-latticed thermoelectric elementsselected from a group of superlattice materials with a ZT greater than0.5 consisting of Bi_(x)Sb_(2−x)Te₃, Bi₂Te_(3−x)Se₃, CdSb, ZnSb, andS_(x)Ge_(1−y).
 46. An electrical apparatus comprising: an armatureincluding stator coils; superconducting rotor coils magnetically coupledto the stator coils; an internal jacket located interior to the armatureand configured to minimize heat conduction from the superconductingrotor coils to the armature; a drive shaft extending through theinternal jacket; a cascade thermoelectric module configured to cool thesuperconducting coils to cryogenic temperatures; a 77 K. transfercoupling in thermal contact with a hot side of the cascadethermoelectric module; a liquid nitrogen refrigeration system configuredto supply liquid nitrogen to the 77 K. transfer coupling; and a sub-77K. transfer coupling in thermal contact with a cold side of the cascadethermoelectric module and with the superconducting coils.
 47. Theapparatus of claim 46, wherein the electrical apparatus comprises atleast one of an electric motor and an electric generator.
 48. Theapparatus of claim 46, wherein the internal jacket comprises a vacuumjacket including adsorbent trapping materials.
 49. The apparatus ofclaim 46, wherein the cascade thermoelectric module comprises: abulk-material-based thermoelectric cooler including plural cascaded coldstages with each successive cascaded cold stage configured to cool to aprogressively lower temperature; and a super-latticed thermoelectriccooler interfaced to said bulk-material-based thermoelectric cooler,wherein the super-latticed thermoelectric cooler comprises at least asingle-staged thin-film thermoelectric device.
 50. The apparatus ofclaim 49, wherein said each successive cascaded cold stage of thebulk-material-based thermoelectric cooler comprises: a heat sourceplate; a heat sink plate operating at an elevated temperature withrespect to the heat source plate; and at least one pair of a n-typebulk-material thermoelectric element and a p-type bulk-materialthermoelectric element connected electrically in series and disposedbetween the heat source and sink plates, each pair of bulk-materialthermoelectric elements electrically connected in series.
 51. Theapparatus of claim 49, wherein each stage of the at least asingle-staged thin-film thermoelectric device comprises: a heat sourceplate; a heat sink plate operating at an elevated temperature withrespect to the heat source plate; and at least one pair of a n-typesuper-lattice thermoelectric element and a p-type super-latticethermoelectric element connected electrically in series and disposedbetween the heat source and sink plates.
 52. An electrical apparatuscomprising: an armature including stator coils; superconducting rotorcoils magnetically coupled to the stator coils; an internal jacketlocated interior to the armature and configured to minimize heatconduction from the superconducting rotor coils to the armature; a driveshaft extending through the internal jacket; a cascade thermoelectricmodule configured to cool the superconducting coils from near roomtemperature to cryogenic temperatures; a heat sink in thermal contactwith a hot side of the cascade thermoelectric module; and a sub-77 K.transfer coupling in thermal contact with a cold side of the cascadethermoelectric module and with the superconducting coils.
 53. Theapparatus of claim 52, wherein the electrical apparatus comprises atleast one of an electric motor and an electric generator.
 54. Theapparatus of claim 52, wherein the internal jacket comprises a vacuumjacket including adsorbent trapping materials.
 55. The apparatus ofclaim 52, wherein the cascade thermoelectric module comprises: abulk-material-based thermoelectric cooler including plural cascaded coldstages with each successive cascaded cold stage configured to cool to aprogressively lower temperature; and a super-latticed thermoelectriccooler interfaced to said bulk-material-based thermoelectric cooler,wherein the super-latticed thermoelectric cooler comprises at least asingle-staged thin-film thermoelectric device.
 56. The apparatus ofclaim 55, wherein said each successive cascaded cold stage of thebulk-material-based thermoelectric cooler comprises: a heat sourceplate; a heat sink plate operating at an elevated temperature withrespect to the heat source plate; and at least one pair of a n-typebulk-material thermoelectric element and a p-type bulk-materialthermoelectric element connected electrically in series and disposedbetween the heat source and sink plates, each pair of bulk-materialthermoelectric elements electrically connected in series.
 57. Theapparatus of claim 55, wherein each stage of the at least asingle-staged thin-film thermoelectric device comprises: a heat sourceplate; a heat sink plate operating at an elevated temperature withrespect to the heat source plate; and at least one pair of a n-typesuper-lattice thermoelectric element and a p-type super-latticethermoelectric element connected electrically in series and disposedbetween the heat source and sink plates.
 58. An electrical apparatuscomprising: an armature including stator coils; superconducting rotorcoils magnetically coupled to the stator coils; an internal jacketlocated interior to the armature and configured to minimize heatconduction from the superconducting rotor coils to the armature; a driveshaft extending through the internal jacket; an integratedthermoelectric cascade module mounted directly against thesuperconducting coils and configured to cool the superconducting coilsfrom near room temperature to cryogenic temperatures; a heat sink atnear room temperature; and a closed loop circulation system configuredto circulate heat transfer fluids between a hot side of the cascadethermoelectric module and the heat sink.
 59. The apparatus of claim 58,wherein the electrical apparatus comprises at least one of an electricmotor and an electric generator.
 60. The apparatus of claim 58, whereinthe internal jacket comprises at least one of: a vacuum jacket includingadsorbent trapping materials; and a radiative heat transfer deviceconfigured to dissipate heat from the cascade thermoelectric modulethrough the vacuum jacket to the armature.
 61. The apparatus of claim58, wherein the integrated cascade thermoelectric module comprises: abulk-material-based thermoelectric cooler including plural cascaded coldstages with each successive cascaded cold stage configured to cool to aprogressively lower temperature; and a super-latticed thermoelectriccooler interfaced to said bulk-material-based thermoelectric cooler,wherein the super-latticed thermoelectric cooler comprises at least asingle-staged thin-film thermoelectric device.
 62. The apparatus ofclaim 61, wherein said each successive cascaded cold stage of thebulk-material-based thermoelectric cooler comprises: a heat sourceplate; a heat sink plate operating at an elevated temperature withrespect to the heat source plate; and at least one pair of a n-typebulk-material thermoelectric element and a p-type bulk-materialthermoelectric element connected electrically in series and disposedbetween the heat source and sink plates, each pair of bulk-materialthermoelectric elements electrically connected in series.
 63. Theapparatus of claim 61, wherein each stage of the at least asingle-staged thin-film thermoelectric device comprises: a heat sourceplate; a heat sink plate operating at an elevated temperature withrespect to the heat source plate; and at least one pair of a n-typesuper-lattice thermoelectric element and a p-type super-latticethermoelectric element connected electrically in series and disposedbetween the heat source and sink plates.